When the MODE pin is connected to logic high, the TPS552872-Q1 works in FPWM mode and the switching frequency is unchanged in light load condition. When the load current decreases, the output of the internal error amplifier decreases as well to reduce the average inductor current down to deliver less power from input to output. When the output current further reduces, the current through the inductor decreases to zero during the switch-off time. The high-side N-MOSFET is not turned off even if the current through the MOSFET is zero. Thus, the inductor current changes its direction after it runs to zero. The power flow is from output side to input side. The efficiency is low in this condition. However, with the fixed switching frequency, there is no audible noise or other problems that might be caused by low switching frequency in light load condition.